1. Field of the Invention
The present invention generally relates to a method and apparatus for reducing pollutants in the exhaust gases produced by the combustion of fuels. More particularly, the invention relates to such a method and apparatus wherein the reduction in pollutants is achieved by introducing hydroxyl radicals "OH" and other free radical intermediaries and oxidizers such as O, H, HO.sub.2 and H.sub.2 O.sub.2 into the precombustion or postcombustion gas stream of a combustion engine.
2. Background
As is well-known in the art, an internal combustion engine draws in ambient air which is mixed with fuel for combustion in a combustion chamber or cylinder and the resulting exhaust gases are expelled. Ignition of the air/fuel mixture in the cylinder is typically achieved by an ignition device, such as, for example, a spark plug or the like, or adiabatic compression to a temperature above the fuel's ignition point.
In certain internal combustion engines, such as for example, gasoline engines commonly in use today, air is inducted via an air intake duct or port which conveys the ambient air to a carburetor or a fuel injection arrangement where the air is mixed with fuel to create an air/fuel mixture. The air/fuel mixture is then conveyed via an intake manifold to the combustion chamber or cylinder of the engine. In diesel-type engines and engines using fuel-injection arrangements, the air and fuel are conveyed separately to the combustion chamber or cylinder of the engine where they are mixed.
After the air/fuel mixture has been burnt, the resulting exhaust gases are expelled from the combustion chamber to an exhaust manifold. The exhaust gases then may be conveyed by an exhaust pipe to the catalytic converter where pollutants are removed.
The flow of air to the combustion chamber, including the flow of the air/fuel mixture if applicable, as used herein is referred to as the precombustion gas stream, and the resulting flow of exhaust therefrom is hereinafter referred to as the postcombustion or exhaust gas stream. As used herein, the precombustion and postcombustion gas streams are collectively referred to as the combustion gas stream.
Internal combustion engines which operate by the controlled combustion of fuels produce exhaust gases containing complete combustion products of carbon dioxide (CO.sub.2) and water (H.sub.2 O) and also pollutants from incomplete combustion such as carbon monoxide (CO), which is a direct poison to human life, as well as unburnt hydrocarbons (HC). Further, due to the very high temperatures produced by the burning of the hydrocarbon fuels followed by rapid cooling, thermal fixation of nitrogen in the air results in the detrimental formation of Nitrogen Oxides (NO.sub.x), an additional pollutant.
The quantity of pollutants varies with many operating conditions of the engine but is influenced predominantly by the air-to-fuel ratio in the combustion cylinder such that conditions conducive to reducing carbon monoxide and unburnt hydrocarbons (a fuel mixture just lean of stoichiometric and high combustion temperatures) cause an increased formation of NO.sub.x, and conditions conducive to reducing the formation of NO.sub.x (fuel rich or fuel lean mixtures and low combustion temperatures) cause an increase in carbon monoxide and unburnt hydrocarbons in the exhaust gases of the engine. Because in modern day catalytic converters NO.sub.x reduction is most effective in the absence of oxygen, while the abatement of CO and HC requires oxygen, preventing the production of these emissions requires that the engine be operated close to the stoichiometric air-to-fuel ratio because under these conditions the use of three-way catalysts (TWC) are possible, i.e., all three pollutants can be reduced simultaneously. Nevertheless, during operation of the internal combustion engine, an environmentally significant amount of CO, HC and NO.sub.x is emitted into the atmosphere.
Although the presence of pollutants in the exhaust gases of internal combustion engines has been recognized since 1901, the need to control internal combustion engine emissions in the United States came with the passage of the Clean Air Act in 1970. Engine manufacturers have explored a wide variety of technologies to meet the requirements of this Act. Catalysis has proven to be the most effective passive system.
Automotive manufacturers have generally employed catalytic converters to perform catalysis. The purpose is to oxidize CO and HC to CO.sub.2 and H.sub.2 O and reduce NO/NO.sub.2 to N.sub.2. Auto emission catalytic converters are typically located at the underbody of the automobile and are situated in the exhaust gas stream of the engine, just before the muffler, which is an extremely hostile environment due to the extremes of temperature as well as the structural and vibrational loads encountered under driving conditions.
Nearly all auto emission catalytic converters are housed in honeycomb monolithic structures with excellent strength and crack-resistance under thermal shock. The honeycomb construction and the geometries chosen provide a relatively low pressure drop and a high geometric surface area which enhances the mass transfer controlled reactions. The honeycomb is set in a steel container and protected from vibration by a resilient matting.
An adherent washcoat, generally made of stabilized gamma alumina into which the catalytic components are incorporated, is deposited on the walls of the honeycomb. TWC technology for simultaneously converting all three pollutants comprises the use of precious or noble metals Pt and Rh, with Rh being most responsible for the reduction of NO.sub.x, although it also contributes to CO oxidation along with Pt. Recently less expensive Pd has been substituted for or used in combination with Pt and Rh. The active catalyst is generally about 0.1 to 0.15% precious or noble metals, primarily platinum (Pt), palladium (Pd) or rhodium (Rh).
Because the exhaust gases of the combustion engine oscillate from slightly rich to slightly lean, an oxygen storage medium is added to the washcoat which adsorbs (stores) oxygen during any lean portion of the cycle and releases it to react with excess CO and HC during any rich portion. Cerium Oxide (CeO.sub.2) is most frequently used for this purpose due to its desirable reduction-oxidation response.
The recent passage of the 1990 amendment to the Clean Air Act requires further significant reductions in the amount of pollutants being released into the atmosphere by internal combustion engines. In order to comply with these requirements, restrictions on the use of automobiles and trucks have been proposed, such as, employer-compelled car pooling, HOV lanes, increased use of mass transit as well as rail lines and similar actions limiting automobile and truck usage at considerable cost and inconvenience.
An alternative to diminished automobile and truck usage is decreasing emissions by increasing the efficiency of the internal combustion engine. This approach will have limited impact since studies show that most of automobile-originated pollution is contributed by only a small fraction of the vehicles on the road, these vehicles typically being older models having relatively inefficient engines and aging catalytic converters which inherently produce a lot of pollution. Any technological improvements to the total combustion process will not be implemented on these older vehicles if they require extensive or expensive modification to the engine or vehicle.
In addition, while considerable gains have been made in recent years to reduce the amount of pollutants in the exhaust gases of the internal combustion engine of vehicles such as automobiles and trucks, it is a considerable technological challenge and expensive to further reduce the amount of pollutants in the exhaust gases of the internal combustion, even though exhaust emissions of automobiles and trucks currently being manufactured do not meet proposed Environmental Protection Agency standards.
In lieu of decreasing exhaust emissions by increasing the efficiency of the internal combustion engine or decreasing the use of automobiles, a further alternative would be to increase the efficiency of the catalytic converter or catalysis. The conversion efficiency of a catalytic converter is measured by the ratio of the rate of mass removal of the particular constituent of interest to the mass flow rate of that constituent into the catalytic converter. The conversion efficiency of a catalytic converter is a function of many parameters including aging, temperature, stoichiometry, the presence of any catalyst poisons (such as lead, sulfur, carbon and phosphorous), the type of catalyst and the amount of time the exhaust gases reside in the catalytic converter.
Attempts to increase the efficiency of catalytic converters has not been sufficiently successful. Modern TWC catalytic converters help, but they are expensive, may have difficulty in meeting the future emission requirements, and have limitations in their performance lifetime. Catalytic converters also suffer from the disadvantage that their conversion efficiency is low until the system reaches operating temperature.